This application claims the priority of German Application No. 100 25 489.6, filed May 23, 2000, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a device for producing metal/ceramic composite materials and to a process for producing such materials.
DE 197 50 599 discloses a process for producing a metal/ceramic composite material, in which a porous ceramic preform is placed into a pressure die-casting die and is infiltrated with liquid aluminium using the pressure die-casting process. In this process, however, the ceramic preform has to be produced, machined, and placed into a pressure die-casting die in a complex way. This may lead to flexural stresses in the preform if the geometric match between the pressure die-casting die and the preform is inadequate. During the infiltration of the preform as part of the pressure die-casting operation, these flexural stresses may cause the preform to fracture and the component to be scrapped.
Accordingly, the present invention is based on the object of providing a device and a process through which the production of a metal/ceramic composite material is simplified and the process reliability is increased.
The object is achieved by a device and a process according to the present invention.
The device according to the present invention is integrated in a pressure die-casting machine or in a squeeze-casting machine, referred to below as xe2x80x9ccasting machinexe2x80x9d to simplify matters. This device has an injection plunger, a casting chamber with an opening, and a die with a runner and a die cavity. The opening of the casting chamber is suitable for filling the casting chamber (1) with a ceramic precursor product for producing a ceramic preform and (2) with a liquid casting metal. This has the resultant advantage that the preform is produced directly in the mold cavity in which it is subsequently infiltrated with the casting metal, so that no further significant remachining is required. The infiltration results in the production of a metal/ceramic composite material or a component made from the composite material. The term xe2x80x9cceramic precursor productxe2x80x9d means a ceramic powder or a ceramic powder mixture which may contain further organic or inorganic auxiliaries.
The powder is transferred, via the opening into the casting chamber, and from there into the runner and the mold cavity, where it is compressed to form the preform. In this context, the term xe2x80x9cpressingxe2x80x9d is understood as meaning compacting of the powder which, depending on the pressure applied, may express itself as a self-supporting structure of intermeshing powder grains or in the form of a loose powder bed. For this operation, a high pressure is required, which can be produced with minimum technical outlay by a shot head. The term shot head is understood as meaning a device which presses pulverulent dry or moist material into a mold by gas pressure and/or mechanical pressure.
The shot head is mounted movably with respect to the opening and can be exchanged for a casting ladle, which is likewise mounted movably with respect to the opening. The casting ladle is used to fill the casting chamber with the liquid casting metal. This device enables the casting chamber to be used to fill the die cavity with different media in succession.
Further, a suction or blowing device can be fitted onto the opening, which device sucks or blows residual powder, in particular which remains in the casting chamber after the powder has been pressed into the mold cavity, out of the casting chamber. In the process, contamination of the casting metal with the powder is prevented, since this would impair the casting performance and the infiltration performance of the casting metal.
In the runner, the die has a blocking slide which, after the die cavity has been filled with the powder, prevents the powder from dropping back into the casting chamber. The closure device of the slide is coupled to the drive of the injection plunger, enabling the slide to be opened when the casting metal is pressed into the runner by the injection plunger.
In conventional pressure die casting, the runner of the die serves, inter alia, to provide further casting metal for recompression after the die cavity has been filled with the casting metal. The device according to the present invention also causes the ceramic powder to be recompressed. In terms of bulk density, this powder takes up a greater volume than a liquid casting metal. For this reason, the runner of the die of the device according to the present invention is designed with a larger volume than is the case with a conventional die. According to the present invention, the volume is between 10% and 30% of the volume of the die cavity, thus ensuring that there is always sufficient powder available for recompression of the preform.
In a further embodiment, a device according to the present invention comprises a die with a mold cavity and a runner which, through a notch, has an opening leading to the die cavity. The die is integrated in a casting machine and, unlike a conventional die, includes at least two openings for filling with material. One opening is used to convey a pressurized powder into the mold cavity and thus to produce a porous ceramic preform in the die cavity. The second opening comprises the notch of the runner and is used to fill the die cavity with a liquid casting metal. With the aid of this device, it is possible to produce the porous ceramic preform directly in the mold cavity and then for this preform to be infiltrated with the casting metal so as to produce a composite material. The opening for filling with powder can be arranged at the most suitable point on the die. The position of this opening depends on the design of the die and of the casting machine. It may be arranged in such a way that the distance which the powder has to cover from the opening into the die cavity is as short as possible.
The powder can be pressed into the die cavity by a shot head. The shot head of a conventional core-shooting machine is recommended for this purpose. This head is usually mounted in a fixed position at the opening of the die, which is advantageous for metering of the powder. Furthermore, the short conveying path allows a relatively high pressure to be applied to the powder, with an associated high degree of compacting.
It is expedient for the die cavity to be closed off from the runner by a blocking slide, in order to ensure a counter-pressure for compacting the powder to form the preform. Furthermore, after the die cavity has been filled with the powder, the opening to the shot head can be closed by a blocking slide. This prevents the casting metal from escaping from the die cavity while it is filling the latter.
To prevent outlet valves that vent the di e cavity from becoming blocked, the device has a filter channel, which is illustrated in FIG. 3 and narrows conically, in front of each of the outlet valves. The passage opening is designed to be sufficiently narrow, as a function of the condition of the powder, for it to become blocked according to the present invention by powder but to be permeable to gaseous media. Furthermore, the filter channel is applied in such a way that its center axis lies in a parting plane between two parts of the die. This has the advantageous effect that the filter channel is cleaned after opening of the die and ejection of the component.
It is possible to place an insert, which fills up certain regions of the mold cavity, in a fixed position in the mold cavity. These regions are not filled while the powder is pressed in and are not represented in the form of a preform. If the geometry of the insert avoids undercuts, as is necessary when designing dies, the insert can be removed after production of the preform. The regions which are now clear and have not been filled by the preform are thus filled by the casting metal alone. As has been described, the preform is infiltrated with the casting metal, so that the composite material is formed, the assumption being that the preform has a self-supporting structure. This has the advantage that the component produced in some regions consists of metal and in some regions consists of composite material. Accordingly, the component has local reinforcements, so that it can optimally satisfy the demands imposed on its materials properties.
A further subject of the present invention is a process for producing a composite material in which a porous ceramic preform is produced in the die cavity of a die and is then infiltrated with a casting metal. This takes place as a result of a powder being pressed into the die cavity by a blowing medium (usually compressed air) via an opening. As a result, a porous ceramic preform, which in a second process step is infiltrated with a casting metal, is formed in the die cavity. The result is a composite material or a component which consists at least partially of the composite material. The advantage of this process is that the preform is produced directly in the die, no machining is required and the preform is optimally adapted to the geometry of the die cavity.
For optimal filling of the die cavity with the powder, it is important for this powder to be fluidized by the compressed air, thus behaving like a Newtonian fluid and moving substantially in the form of a laminar flow.
Further, the blowing medium for the powder is pressed out of the die cavity through outlet valves. To prevent the outlet valves from becoming blocked, they are in each case protected by a filter channel.
While the die cavity is being filled with the powder, it is appropriate for all the further openings which the die cavity may have and which are not used for venting the die cavity (e.g., filter channels) to be closed, so that the necessary counter-pressure for compacting the powder can be built up and the powder cannot escape from the die cavity. It is appropriate for the opening of the die cavity through which the powder is introduced to be closed during infiltration of the preform with the casting metal, provided that filling with the powder and with the casting metal do not take place through the same opening of the die cavity.
The optimum grain structure of the powder is as far as possible of spherical nature for good fluidization of the powder. Another advantage is that the grain diameters are scattered about a mean in similar manner to a Gaussian distribution and thus have a monomodal distribution. In general, it is possible to manufacture machine parts with a tolerance of 50 xcexcm. To minimize die wear, the minimum grain size of the powder should be over 50 xcexcm, in particular over 100 xcexcm.
In the context of the process employed, it is expedient to use casting metals which are also employed in the pressure die-casting process or squeeze-casting process, i.e. alloys which are based on aluminium or magnesium.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.